Cutting a large special occasion cake might seen as daunting as loosening the Gordian Knot. One tip is to use a large serrated knife dipped before each cut into hot water. This method is especially useful for a buttercream based frosting. Still, the question is where to cut. I have attached below the guides I give to my pastry students. The goals is always to get uniform portions that still look good on the plate.
Instagram has a posting with a new-to-me method for cutting a cake. Using a small cutting board, a large section is carved off of the larger cake and sectioned into portion sizes. The warm knife method still applies.
I have another method for round cakes that involves making a circular cut around the outer edge, about four inches on and then cutting portions as wedges, Repeat the method for a larger cake.